1. Field of the Invention
The present invention relates generally to the field of cables for optical fibers, and more particularly to an optical fiber cable designed for easy fiber access while maintaining fiber strength.
2. Description of the Prior Art
With the advent of local area networks and the relative broadband capabilities of fiber optic links, it has become commonplace for new communication systems to include fiber optic capabilities. In the use of optical fibers, it is important to provide physical protection for the fibers in view of the fragile nature of glass optical fibers. This is not to imply that optical fibers are not reliable over long periods of time, for such is not the case. Optical fibers, when properly protected, have excellent service lifetime. However, proper protection involves providing a cable which shields the fiber from physical stresses as well as extreme environmental conditions. The potential for optical fiber deterioration in wet environments is one example of the environmental conditions which must be avoided for reliable fiber service over extended periods of time.
A significant hazard to fiber service lifetime is stress, including tensile, compressive and torsional stresses. Optical fibers tend to develop microcracks when exposed to various stresses or short radius bends. Thus, it is desirable to provide a cable for the fibers which minimizes the stress and bending to which the fiber will be exposed. Microcracks eventually increase in size to the point where the crack interferes with optical transmission quality and causes diffusion of the light. Fibers having excessive interference are not suitable for broad bandwidth transmission and hence will have to be replaced by substitution of a spare fiber already in the cable or replacement of the entire cable.
In order to reduce fiber tension, it is common to provide a slight overlength of fiber in the cable so that as the cable is stretched the overlength is used to avoid tension on the fiber. The fiber is usually disposed loosely in the cable so that it can freely bend to absorb any cable compression without compressing the fiber. However, as mentioned above, microcracks also develop if the fiber is bent with too small a radius. Thus, care must be taken to provide just enough overlength to avoid fiber tension while avoiding too much overlength so that in cable compression the fiber is sharply bent so as to develop microcracks.
In local area network applications, each fiber must be tapped numerous times to extract and inject a light signal. It is not uncommon to require as many as 80 taps for each individual fiber. It is well known in the optical fiber art that each time a fiber is cut and re-spliced to effect a tap, significant attenuation of the optical signal is experienced at the splice. As the number of taps increase, the attenuation becomes unacceptable.
Accordingly, techniques have been developed to effect a tap without cutting or splicing the fiber, thereby eliminating the splice loss. An optical tapping technique commonly implemented requires bending the optical fiber until it has a radius of curvature which causes optical energy to exit from the core of the fiber being tapped and into the cladding layer where it can be directed to a coupler. Couplers of this type are well known and widely available from a variety of sources, such as RAYNET Corp. and are referred to as LID couplers, indicating that the couplers are intended for Local Injection and Detection (LID) of optical energy.
In cables having a plurality of fibers it has always proven difficult to identify one fiber from another. This difficulty is sometimes addressed by providing a color coding in the form of ink applied to the exterior of the optical fiber. This solution to the fiber identity problem created another difficulty in the use of fiber optic cables. Tapping the fiber using an LID coupler as described above requires that the glass core of the fiber be permitted to release some of the optical energy to the cladding from where it is directed to the coupler. The inking which was provided for fiber identity must be removed or it would block or reduce light transfer. Removal of the inking was accomplished by a quick acid etch. This worked well for removal of the ink, but has been found to be a cause of deterioration of the fiber at the location of the etch.
The deterioration of the fiber at the location of the etch caused a further complication to arise in fiber tapping arrangements as described above. Since the optical energy to be tapped must somehow be extracted from the fiber, it is necessary to cause the optical energy to be concentrated in the cladding, or at least to exit in part from the core, at the point where the tap is to occur. This is accomplished by introducing a sharp bend into the fiber at the tap location. The bend in the fiber, however, introduces stress into the fiber. This stress, in conjunction with the deterioration commenced as a result of the etch, has been found to be a source of fiber failure. Thus, the use of fiber inking for fiber identification has a significant drawback. These two detrimental conditions, the acid etch and the bending stresses, co-operate at the location of the fiber tap to deteriorate fiber quality.
As may be apparent from the above description of the fiber tapping technique commonly employed, there is a need for excess fiber length relative to the length of the cable. This extra length is needed in order to accommodate the introduction of a bend into the fiber for purposes to tapping the optical signal without cutting the cable. The extra length is usually provided by laying the fiber in the cable in a reverse helical manner. When fibers are laid in this manner the fiber can be unwound from a helical reversing point to provide the extra length.
Most optical fiber cables use radial strength components disposed between jacketing layers. This structure presents another problem when fibers within the cable must be accessed midway the cable. The first difficulty that is presented is that the strength members must be cut and removed over a finite length of the cable. This in itself is a time-consuming task. The second difficulty is that if the cable is under any stress, removal of the strength members will cause the stress to be transferred to the remaining components, including the fibers.
Providing a larger strength member at the center of the cable greatly increases the cable diameter which adds to its cost and is unsuitable, especially for local area network applications where a thin, unobtrusive cable is desired. Thus, providing easy fiber access while maintaining cable strength and minimal size is a difficult problem.
Even with the current level of understanding of the conflicting needs associated with optical cables, there has not previously been a cable design which provides ideal service in applications where multiple optical taps may be necessary. A comprehensive illustration of the prior art may be obtained by reference to the following U.S. Pat. Nos.: 4,038,489; 4,195,468; 4,227,770; 4,361,381; 4,389,088; 4,401,366; 4,585,305; and 4,804,245